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Defects on the Model 31 Carbine, bolt sleeves, locking piece, bayonet (Created: 25.06.2024)

The defects discussed in this article are based on findings from expert commissions in the 1950s. However, defects can occur in all weapons, depending on their usage and age. Every weapon must be regularly subjected to a visual inspection!
For the Modell 1931 Carbine, the reports (see Attachement) highlight the following points of particular concern:
- Bolt sleeves: compressions
- Bolt sleeves: hairline cracks in the bolt lugs
- Cocking piece: cracks or breaks
For Modell 1931 Carbine within the serial number range between 750000 and 850000 (note by the author: probably 860000) that are actively used, it is necessary to check whether the bolt sleeve has been replaced during an inspection (i.e., numbered with an electric engraver). This applies to sleeves made from replacement material ("Mo") as well as those from "normal" material ("CN")! This is because some of the "CN" sleeves within this serial number range were insufficiently hardened by mistake (see the result of a bolt sleeve with serial number 785750: Picture).
Defects on the Modell 1931 Carbine
In the mid-1950s, various expert commissions addressed improvements to the Modell 1931 Carbine. The reason for this was some defects that had emerged over time. At that time, the Swiss arms industry and the federal government set very high quality standards.
The Swiss Federal Archives contain various documents on this topic. This article is largely based on this document: Attachement
Briefly listed, improvements for the following defects were discussed:
- M1: Breaks in the cocking piece of the bolt
- M2: Compressed bolt sleeves
- M3: Cracks in the bolt lugs of the bolt sleeves
- M4: Dagger bayonets or dagger hooks
Defect M1: Breaks in the cocking piece of the bolt
The cocking piece of the bolt tended to break. If this occurs, it can result in accidents causing personal injury. Until 1937, the bolt (bolt handle, bolt slider) was pressed from steel, not hardened, and not blued (i.e., uncoated). From 1937, bolt handles were stamped from sheet steel, hardened, and mounted blued. Breaks in the cocking piece can occur in both versions, depending on the treatment of the carbine and the number of shots fired. Incidentally, already manufactured uncoated bolt handles were partially used in private Modell 1931 Carbine.
Defect M2: Compressed bolt sleeves, M3: Cracks in the bolt lugs
During the war years, so-called replacement materials were necessarily and consciously used (see Attachement, page 6, question 4b, and page 8, question 8a). Chromium-molybdenum steel (marked Mo) was used instead of chromium-nickel steel (marked CN) for various parts. In the bolt sleeve, the part that is most stressed, this could lead to problems, especially compressions and cracks.
The report (see Attachement, page 6, question 4b) provides the answers for the problems:
1. During the war, replacement steel had to be used, which had to be kept low in strength due to its tendency to brittleness. In 1946, the W+F demanded from the KMV that the too soft sleeves be replaced.
2. Sleeves made from the normally used steel, which were not properly hardened due to a mix-up in material designation.
The report (see Attachement, page 6, question 5, at the bottom) also states the following section:
"The sleeves made with replacement steel during the war were used in carbines with serial numbers approximately 750,000 - 850,000. These sleeves were intentionally hardened too softly at the time because it was preferred to replace a sleeve rather than risk breaking a bolt. Too soft and therefore compressed sleeves can lead to the bolt jamming. They must be replaced. When replacing, the cartridge chamber depth must also be checked, which may require the carbine to be returned to an armory. It will be necessary to generously replace the sleeves made with replacement steel if they are defective."
The report also states (see Attachement, page 8, question 8a) that "The sleeves made with this material were adequate for wartime," as "they withstood high numbers of shots despite the deformations that occurred."
The report thus indicates that both Mo sleeves and CN sleeves in the range of 750000 to 850000 (note by the author: probably 860000) are affected by too soft hardening! This can result in compression of the sleeve or hairline cracks in the bolt lugs, depending on usage and the number of shots fired. Something like this must have happened to the CN sleeve with serial number 785750: Picture, Picture, Picture).
Defect M4: Dagger bayonet or dagger hook
This defect occurs when shooting with a mounted bayonet. This is irrelevant today and is therefore not pursued further.
Marking of bolt sleeves with an electric engraver
Replacement bolt sleeves left the factory fully hardened, making it impossible to stamp the number. For this reason, the weapon numbers were hand-engraved with an electric engraver (example: Picture). Sleeves are also known that received the weapon number using a engraving machine.
The cocking piece must be checked regularly.
Bolt sleeves must be regularly inspected for compressions and hairline cracks, especially on Modell 1931 Carbine within the range of 750000 to 850000 (note by the author: probably 860000).
A CN sleeve within the range of 750000 to 850000 (note by the author: probably 860000) with the original stamped number may be incorrectly hardened and should be treated like a Mo sleeve.
As a rule of thumb: if the sleeve is numbered with an electric engraver, it has been replaced with a sleeve made of properly hardened CN steel.
What happened next with the report?
The report was published in 1956. The introduction of an assault rifle was imminent, so no more design changes were implemented. The Modell 1955 Sniper Carbine has already seen some design improvements and may represent the end of the evolution of Swiss straight-pull bolt-action rifles.